Technical Field
The present invention relates to a configuration of a hybrid vehicle and a control method thereof.
Related Art
Recently, hybrid vehicles combining an engine and a motor generator are widely used. Such a hybrid vehicle often uses a vehicle drive system including a planetary gear mechanism which distributes motive power transmitted from an engine to a first motor generator and a vehicle drive shaft; and a second motor generator which transmits motive power to the vehicle drive shaft. Because the planetary gear mechanism cannot rotate the vehicle drive shaft in the reverse direction which is opposite to the rotation direction of the engine, a hybrid vehicle having such a vehicle drive system performs reverse travel by rotating the second motor generator connected to the vehicle drive shaft in the reverse direction.
Because the second motor generator is driven by using electric power from a battery, when the residual capacity of the battery is not enough, the reverse travel is performed by rotating the second motor generator in the reverse direction while driving the first motor generator by the engine to charge the battery with the generated electric power. A method is suggested to generate sufficient electric power to charge the battery when travelling in reverse by setting a higher rotation speed of the engine for a lower residual capacity of the battery. (For example, JP 2004-56922 A.)
Further, a method to control an engine and a motor generator is suggested in which, when travelling in reverse while charging a battery by using power from the engine, a higher electric power is assigned to be charged for a gentle uphill run, while a lower electric power is assigned to be charged for a steep uphill run. In this method, the power requested in accordance with the electric power assigned to be charged is transmitted from the engine such that the requested torque is transmitted to the drive shaft. (For example, JP 2010-195255 A.)
A method to increase drive torque of the second motor generator in the reverse direction by driving the engine using the first motor generator is also suggested (For example, JP 2011-218827 A).
However, in the conventional method disclosed in JP 2004-56922 A, when the battery voltage or the residual capacity of the battery is decreased due to, for example, a low temperature, drivability is deteriorated because the engine rotation speed becomes higher regardless of the driver's intention. Further, because the engine is maintained at a higher rotation speed for a lower battery voltage or residual capacity of the battery, a problem occurs that the system efficiency of the hybrid vehicle is deteriorated, resulting in a shorter drivable distance.
In a conventional control method disclosed in JP 2010-195255 A, when the steepness of an uphill run is large, demanding a high output, the electric power which can be charged to the battery is reduced. Therefore, in reverse travel on a steep uphill run, there is a problem that, because the driving force of the second motor generator decreases due to a decrease in the residual capacity of the battery, the drivability is deteriorated due to insufficient acceleration. A conventional method disclosed in JP 2011-218827 A has a problem that because the drive torque of the second motor generator is increased while driving the engine using the first generator, an energy loss becomes larger and the system efficiency of the hybrid vehicle is deteriorated.